JP4834463B2 - Noise detecting device and AM broadcast receiving device - Google Patents

Description

  The present invention relates to a noise detection device and an AM broadcast receiving device capable of accurately detecting short-cycle noise and effectively removing short-cycle noise emitted from a streetcar, a power transmission line, or the like.

  FIG. 6 shows a conventional configuration of the AM broadcast receiving apparatus 1 having a noise removal function. As shown in the figure, this AM broadcast receiving apparatus 1 includes an antenna 5 that receives broadcast radio waves, an FE unit 10 that performs tuning and high frequency amplification, an IF unit 12 that performs frequency conversion and intermediate frequency amplification, and is converted into an intermediate frequency. A detection unit 13 that performs AM detection of the received signal, a noise detection device 14 that detects the presence or absence of noise and outputs a noise detection signal, a noise canceller 15 that removes noise contained in the reception signal, and an amplification unit 16 that amplifies the audio signal The speaker 17 is configured to output sound.

FIG. 7 shows the configuration of the noise detection device 14. The noise detection device 14 includes a filter 71 that limits a band of a detection signal output from the detection unit 13, a low-pass filter 72 that smoothes a signal output from the filter 71, and a signal output from the low-pass filter 72 as a predetermined threshold value. And a comparator 73 that outputs the noise detection signal when the level of the smoothed signal exceeds a threshold value.
JP 2001-186031 A

  By the way, for example, in the case of the on-board AM broadcast receiving apparatus 1, the voice quality deteriorates due to relatively short-period pulse noise (hereinafter referred to as short-period noise) emitted from a streetcar, a power transmission line, or the like. Therefore, a mechanism for effectively removing short-period noise is necessary.

  However, the AM broadcast receiving apparatus 1 having the conventional configuration described above cannot always effectively remove short-period noise. That is, the conventional noise detection device 14 detects the presence or absence of noise based on a broadband detection signal output from the detection unit 13 as shown in FIG. When close to noise caused by a station (hereinafter referred to as adjacent interference wave), the noise canceller 15 frequently operates due to the adjacent interference wave, and the sound quality of the AM broadcast receiving apparatus 1 is deteriorated.

  The present invention has been made in view of such a problem, and an object of the present invention is to provide a noise detection device capable of detecting short-period noise with high accuracy and effectively removing short-period noise.

  In order to achieve the above object, a main invention of the present invention is a noise detection device for detecting the presence or absence of noise included in a received signal, which does not include the frequency band of the received signal and is the center of the received signal. A first band selector for selecting a signal in a first frequency band higher than the frequency, and a signal in a second frequency band that does not include the frequency band of the received signal and is lower than the center frequency of the received signal The second band selecting unit that selects the first signal, the first signal strength selected by the first band selecting unit, and the second signal strength selected by the second band selecting unit are compared. A signal selection unit that selects a signal having a lower intensity, a comparison unit that compares the intensity of the signal selected by the signal selection unit with a predetermined threshold, and outputs a signal according to the comparison result; Is included.

  The noise detection device of the present invention compares the intensity of the first signal selected by the first band selection unit with the intensity of the second signal selected by the second band selection unit, and has a weak intensity. By selecting one of the signals, a frequency band that is less affected by adjacent interference waves is selected, the intensity of the selected signal is compared with a predetermined threshold value, and a signal corresponding to the comparison result is output. For this reason, it is possible to detect the presence or absence of short-period noise with a signal in a frequency band where the influence of adjacent interference waves is small, and short-period noise is effectively prevented by using the signal output from the noise detection device of the present invention. Can be removed.

  If the received signal is a signal based on a broadcast radio wave transmitted from a broadcast station, broadcast at a frequency adjacent to the frequency at which the desired broadcast station is broadcasting in the first frequency band or the second frequency band. One of the first frequency band and the second frequency band not including the center frequency of the broadcast radio wave of the other broadcast station. The signal in the frequency band is always selected by the comparison unit. For this reason, in any of the first frequency band and the second frequency band, a band that does not include the center frequency of the broadcast radio wave of another broadcast station that is broadcasting at a frequency adjacent to the frequency of the broadcast station desired to receive. It is preferable to set to.

  According to the present invention, it is possible to accurately detect short-period noise and effectively remove short-period noise.

  Hereinafter, embodiments of the present invention will be described in detail. FIG. 1 shows the configuration of a superheterodyne AM broadcast receiving apparatus 1 described as an embodiment of the present invention. As shown in the figure, the AM broadcast receiving apparatus 1 includes an antenna 5 that receives broadcast radio waves, an FE unit 10 (FE: Front End) that performs tuning and high-frequency amplification of the received signal, and signals output from the FE unit 10. A / D converter 11 that samples and converts to a digital signal, IF unit 12 that performs conversion and intermediate frequency amplification of the received signal, detection unit 13 that performs AM detection, noise presence / absence detection, noise detection signal A noise detection device 14 that outputs noise, a noise canceller 15 that performs processing to remove noise, an amplification unit 16 that amplifies an audio signal, and a speaker 17 that outputs audio based on a signal output from the noise canceller 15. Yes. The noise detection device 14 and the noise canceller 15 are realized using, for example, a DSP (Digital Signal Processor).

  The noise canceller 15 uses the noise detection signal as a signal that defines the timing for performing interpolation processing on the noise portion in the received signal. The noise canceller 15 effectively removes noise from the received signal by specifying a period in which noise exists based on the noise detection signal and interpolating the period with a waveform based on audio signal waveforms before and after the specified period. In the AM broadcast receiving apparatus 1 of the present embodiment, the noise detection device 14 detects the presence or absence of noise based on the IF signal output from the IF unit 12.

  FIG. 2 shows the configuration of the noise detection device 14. The noise detection device 14 receives the first signal selected by the first band selection unit 142, the second band selection unit 143, and the first band selection unit 142 to which the signal output from the IF unit 12 is input. The signal selection unit 144 that compares the intensity with the intensity of the second signal selected by the second band selection unit 143 and selects and outputs the lower one, and is set according to the electric field strength of the IF signal The intensity of the signal output from the signal selection unit 144 by comparing the intensity of the signal output from the threshold control unit 145 and the signal selection unit 144 with the predetermined threshold output from the threshold control unit 145 The comparator 146 is configured to output a noise detection signal when is larger than the threshold value.

  Further, the first band selection unit 142 includes a first bandpass filter 1421, a first fullwave rectification unit 1422 that performs fullwave rectification on the signal band-limited by the first bandpass filter 1421, and a first allband rectifier 1422. The first low-pass filter 1423 that smoothes the signal rectified by the wave rectifying unit 1422 is configured. The second band selection unit includes a second band-pass filter 1431, a second full-wave rectification unit 1432 that performs full-wave rectification on the signal band-limited by the second band-pass filter 1431, and a second band-pass filter 1431 A second low-pass filter 1433 that smoothes the signal rectified by the full-wave rectification unit 1432 is included.

  Here, the band of the first bandpass filter 1421 of the first band selection unit 142 does not include the frequency band of the IF signal (received signal) and has a first frequency band higher than the center frequency of the IF signal. It is set to select a signal. The band of the second bandpass filter 1431 of the second band selection unit 143 does not include the frequency band of the IF signal (received signal) and has a second frequency band lower than the center frequency of the IF signal. It is set to select a signal. Further, the first frequency band and the second frequency band are set to have a symmetrical relationship with respect to the center frequency of the IF signal (reception signal). Furthermore, the first frequency band and the second frequency band are set to frequency bands that do not include the center frequency of the broadcast radio wave of another broadcast station that is broadcasting at a frequency adjacent to the frequency of the broadcast station.

  FIG. 3 is a setting example of the first band-pass filter 1421 and the second band-pass filter 1431 when the AM broadcast receiving apparatus 1 is an AM medium-wave radio broadcast receiving apparatus that receives radio waves from a Japanese broadcasting station. FIG. 6 is a frequency response curve of the first band-pass filter 1421 and the second band-pass filter 1431. The figure shows the case where the sampling frequency of the A / D converter 11 is set to 110.25 kHz and the center frequency of the IF signal is set to 27.5625 kHz. The first frequency band and the first frequency band of the first bandpass filter 1421 are shown in FIG. The second frequency band of the second bandpass filter 1431 is set to a band that does not include the frequency band (27.5625 kHz ± 7.5 kHz) of the IF signal.

  Here, the signal strength of adjacent interference waves is generally stronger than the signal strength of short-period noise. The adjacent interference wave appears strongly in one of the first frequency band higher than the center frequency of the received signal or the second frequency band lower than the center frequency of the received signal. Therefore, the intensity of the first signal selected by the first band selection unit 142 is compared with the intensity of the second signal selected by the second band selection unit 143, and the By selecting the weaker one, it is possible to select a band that is less affected by adjacent interfering signals and compare the signal strength of this band with a threshold value to detect short-period noise with high accuracy. Can be effectively removed.

  In addition, the first frequency band or the second frequency band includes the center frequency of the broadcast radio wave of another broadcast station that is broadcasting at a frequency adjacent to the frequency at which the broadcast station desired to receive broadcasts. If so, the comparison unit 146 always selects a signal in one of the first frequency band and the second frequency band that does not include the center frequency of the broadcast radio wave of another broadcast station. . For this reason, in any of the first frequency band and the second frequency band, a band that does not include the center frequency of the broadcast radio wave of another broadcast station that is broadcasting at a frequency adjacent to the frequency of the broadcast station desired to receive. Is set.

  That is, in Japan, the frequency of each broadcasting station is assigned at 9 kHz intervals. Therefore, in the example shown in FIG. 3, the first frequency band of the first bandpass filter 1421 and the second bandpass filter 1431 are used. The second frequency band is set to ± 9 kHz, ± 18 kHz, ± 27 kHz, etc., of the center frequency (0...) Of the other adjacent broadcasting stations with respect to the center frequency (27.5625 kHz) of the IF signal (received signal). 5625 kHz, 9.5625 kHz, 18.5625 kHz, 36.5625 kHz, 45.5625 kHz, 54.5625 kHz).

  In FIG. 3, although the center frequency (27.5625 kHz) of the IF signal is set to ½ of the Nyquist frequency (55.125 kHz), the bandpass filter can be easily designed with such a setting. It becomes.

<Description of processing>
Next, processing performed by the noise detection device 14 will be described in detail with reference to the waveform diagrams shown in FIGS. 4A to 4E.

  FIG. 4A shows an example of the waveform of an IF signal including short-period noise that is input from the IF unit 12 to the noise detection device 14, and FIG. 4B shows the frequency characteristics of the IF signal shown in FIG. 4A. A signal input from the IF unit 12 to the noise detection device 14 is input to the first band selection unit 142 and the second band selection unit 143. The signal input to the first band selection unit 142 is band-limited by the first bandpass filter 1421. The signal input to the second band selection unit 143 is band-limited by the second bandpass filter 1431. FIG. 4C shows a waveform (Low side) of a signal output from the first bandpass filter 1421 (High side) and a signal output from the second bandpass filter 1431.

  The signal band-limited by the first bandpass filter 1421 is full-wave rectified in the first full-wave rectification unit 1422. In addition, the signal band-limited by the second bandpass filter 1431 is full-wave rectified in the second full-wave rectification unit 1432. FIG. 4D shows a signal output from the first full-wave rectification unit 1422 (High side) and a signal output from the second full-wave rectification unit 1432 (Low side).

  The signal that has been full-wave rectified by the first full-wave rectification unit 1422 is smoothed by the first low-pass filter 1423. The signal that has been full-wave rectified by the second full-wave rectifier 1432 is smoothed by the second low-pass filter 1433. FIG. 4E shows a signal output from the first low-pass filter 1423 (High side) and a signal output from the second low-pass filter 1433 (Low side).

  The signal selection unit 144 compares the intensity of the signal output from the first low-pass filter 1423 with the intensity of the signal output from the second low-pass filter 1433, and selects and outputs the one having the lower intensity. In FIG. 4E, the intensity of the signal output from the second low-pass filter 1433 is weaker. In this case, the signal output from the second low-pass filter 1433 is selected.

  The comparison unit 146 compares the intensity of the signal output from the signal selection unit 144 with the threshold value output from the threshold control unit 145, and outputs a signal (noise detection signal) corresponding to the comparison result. Here, the noise detection signal takes one logical value (High or Low) if the intensity of the signal output from the signal selection unit 144 is larger than the threshold output from the threshold control unit 145, and the signal selection unit If the intensity of the signal output from 144 is equal to or lower than the threshold output from the threshold control unit 145, the other logical value is obtained. The noise canceller 15 performs interpolation processing for a period in which noise exists based on the noise detection signal, thereby effectively removing short-period noise from the received signal.

<Threshold setting method>
The threshold value that is compared with the intensity of the signal selected by the signal selection unit 144 by the comparison unit 146 is set to an appropriate value for determining the presence or absence of short-period noise based on an experimental value or the like. In addition, for example, a memory for storing data is provided in the noise detection device 14, and the relationship between the electric field strength of the received signal and the signal strength of white noise included in the received signal (see FIG. 5) is stored in the memory. The threshold control unit 145 may set the threshold so that the threshold is slightly larger than the white noise signal intensity corresponding to the current electric field intensity of the received signal. By setting the threshold value to such a value, it is possible to prevent the noise detection device 14 from detecting white noise as short-period noise. Note that the electric field strength of the received signal is obtained from, for example, an S meter circuit provided in the IF unit 12 or the like.

  As mentioned above, although one Embodiment of this invention was described in detail, description of Embodiment mentioned above is for making an understanding of this invention easy, and does not limit this invention. It goes without saying that the present invention can be changed and improved without departing from the gist thereof, and that the present invention includes equivalents thereof.

It is a figure which shows the structure of AM broadcast receiver 1 of the superheterodyne system by one Embodiment of this invention. It is a figure which shows the structure of the noise detection apparatus 14 by one Embodiment of this invention. It is a frequency response curve of the 1st band pass filter 1421 and the 2nd band pass filter 1431 shown as an example of setting of the 1st band pass filter 1421 and the 2nd band pass filter 1431 by one embodiment of the present invention. It is a wave form diagram explaining the process performed by the noise detection apparatus 14 by one Embodiment of this invention, and is an example of the waveform of IF signal containing short period noise. It is a figure explaining the process performed by the noise detection apparatus 14 by one Embodiment of this invention, and is the frequency characteristic of IF signal shown to FIG. 4A. It is the waveform of the signal output from each of the 1st band pass filter 1421 and the 2nd band pass filter 1431 by one Embodiment of this invention. 4 is a waveform of a signal output from each of a first full-wave rectification unit 1422 and a second full-wave rectification unit 1432 according to an embodiment of the present invention. It is a waveform of the signal output from each of the 1st low-pass filter 1423 and the 2nd low-pass filter 1433 by one Embodiment of this invention. It is a figure which shows the relationship between the electric field strength of the received signal by one Embodiment of this invention, and the signal strength of the white noise contained in a received signal. It is a figure which shows the structure of AM broadcast receiver 1 which has a noise removal function. It is a figure which shows the structure of the noise detection apparatus 14 shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 AM broadcast receiver 5 Antenna 10 FE part 11 A / D converter 12 IF part 13 Detection part 14 Noise detection apparatus 142 1st band selection part 1421 1st band pass filter 1422 1st full wave rectification part 1423 1st Low-pass filter 143 Second band selection unit 1431 Second band-pass filter 1432 Second full-wave rectification unit 1433 Second low-pass filter 144 Signal selection unit 145 Threshold control unit 146 Comparison unit 15 Noise canceller 16 Amplification unit 17 Speaker

Claims (6)

A noise detection device that detects the presence or absence of noise included in a received signal based on broadcast radio waves transmitted from a broadcast station ,
A first band selection unit that selects a signal in a first frequency band that does not include the frequency band of the received signal and is higher than the center frequency of the received signal;
A second band selection unit that selects a signal in a second frequency band that does not include the frequency band of the received signal and is lower than the center frequency of the received signal;
The intensity of the first signal selected by the first band selection unit is compared with the intensity of the second signal selected by the second band selection unit, and the signal having the lower intensity is selected. A signal selector to perform,
The intensity of the signal selected by the signal selecting unit with a predetermined threshold value, look including a comparing section for outputting a signal corresponding to the result of said comparison,
The first frequency band and the second frequency band are the center frequency of the broadcast radio wave of another broadcast station that is broadcasting at a frequency adjacent to the frequency of the broadcast station, and the broadcast of the other broadcast station, respectively. A noise detection apparatus, wherein the noise detection apparatus is set to a band that does not include a frequency band higher than the center frequency of the radio wave and a frequency band lower than the center frequency of the broadcast radio wave of the other broadcasting station . The noise detection device according to claim 1,
The first band selection unit or the second band selection unit includes a band-pass filter, a full-wave rectification unit that full-wave rectifies a signal that has passed through the band-pass filter, and a signal output from the full-wave rectification unit. And a low-pass filter that smoothes the noise. The noise detection device according to claim 1,
The signal according to the comparison result output from the comparison unit is a signal that defines a timing for performing an interpolation process for a noise portion in the reception signal. The noise detection device according to claim 1,
A noise detection apparatus comprising: a threshold control unit that sets the threshold according to the electric field strength of the received signal. The noise detection device according to claim 4 ,
The threshold control unit stores the relationship between the electric field strength of the received signal and the signal strength of white noise included in the received signal,
The threshold value control unit sets the threshold value to a value larger than the signal strength of white noise corresponding to the current electric field strength of the received signal. A front-end unit that performs high-frequency amplification of a received signal based on broadcast radio waves transmitted from a broadcast station ;
An IF unit for converting the received signal to an intermediate frequency;
A detection unit that performs detection of the received signal,
A noise detection device that outputs a noise detection signal that is a signal indicating the presence or absence of noise included in the received signal;
A noise canceller that performs processing for removing noise included in the received signal based on the noise detection signal;
The noise detection device includes:
A first band selection unit that selects a signal in a first frequency band that does not include the frequency band of the received signal and is higher than the center frequency of the received signal;
A second band selection unit that selects a signal in a second frequency band that does not include the frequency band of the received signal and is lower than the center frequency of the received signal;
The intensity of the first signal selected by the first band selection unit is compared with the intensity of the second signal selected by the second band selection unit, and the signal having the lower intensity is selected. A signal selector to perform,
The intensity of the selected signal by the signal selecting unit with a predetermined threshold value, look including a comparing section for outputting a signal corresponding to the result of said comparison,
The first frequency band and the second frequency band are the center frequency of the broadcast radio wave of another broadcast station that is broadcasting at a frequency adjacent to the frequency of the broadcast station, and the broadcast of the other broadcast station, respectively. An AM broadcast receiving apparatus, wherein the AM broadcast receiving apparatus is set to a band that does not include a frequency band higher than the center frequency of the radio wave and a frequency band lower than the center frequency of the broadcast radio wave of the other broadcasting station .